Duncan's position puts him in possible conflict with Obama, who has not publically backed same-sex marriage. Obama says his position on the matter is evolving.

In an interview Monday on MSNBC, Duncan was asked if he believes same-sex couples should be able to legally marry. He responded: "Yes, I do."

Duncan's revelation follows Vice President Joe Biden's assertion this weekend that he is "absolutely comfortable" with same-sex married couples getting the same rights at heterosexual married couples. Gay rights advocates interpreted Biden's words as an endorsement of gay marriage, while the White House insisted the vice president had not broken ranks with the president.